<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>星空特效</title>
  </head>
  <body>
    <div>
      <canvas id="canvas"></canvas>
      <canvas id="snow"></canvas>
      <div class="am-g" style="position: fixed; bottom: 0px">
        <div class="am-u-sm-12">
          <div style="z-index: 9999" id="player" class="aplayer"></div>
        </div>
      </div>
    </div>
    <style type="text/css">
      canvas {
        position: fixed;
        width: 100%;
        height: 100%;
        z-index: -1;
      }
    </style>
    <script type="text/javascript">
      var canvas = document.getElementById("canvas"),
        ctx = canvas.getContext("2d"),
        w = (canvas.width = window.innerWidth),
        h = (canvas.height = window.innerHeight),
        hue = 217,
        stars = [],
        count = 0,
        maxStars = 1300; //星星数量
      var canvas2 = document.createElement("canvas"),
        ctx2 = canvas2.getContext("2d");
      canvas2.width = 100;
      canvas2.height = 100;
      var half = canvas2.width / 2,
        gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
      gradient2.addColorStop(0.025, "#CCC");
      gradient2.addColorStop(0.1, "hsl(" + hue + ", 61%, 33%)");
      gradient2.addColorStop(0.25, "hsl(" + hue + ", 64%, 6%)");
      gradient2.addColorStop(1, "transparent");
      ctx2.fillStyle = gradient2;
      ctx2.beginPath();
      ctx2.arc(half, half, half, 0, Math.PI * 2);
      ctx2.fill();
      function random(min, max) {
        if (arguments.length < 2) {
          max = min;
          min = 0;
        }
        if (min > max) {
          var hold = max;
          max = min;
          min = hold;
        }
        return Math.floor(Math.random() * (max - min + 1)) + min;
      }
      function maxOrbit(x, y) {
        var max = Math.max(x, y),
          diameter = Math.round(Math.sqrt(max * max + max * max));
        return diameter / 2;
        //星星移动范围，值越大范围越小，
      }
      var Star = function () {
        this.orbitRadius = random(maxOrbit(w, h));
        this.radius = random(60, this.orbitRadius) / 8;
        //星星大小
        this.orbitX = w / 2;
        this.orbitY = h / 2;
        this.timePassed = random(0, maxStars);
        this.speed = random(this.orbitRadius) / 50000;
        //星星移动速度
        this.alpha = random(2, 10) / 10;
        count++;
        stars[count] = this;
      };
      Star.prototype.draw = function () {
        var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
          y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
          twinkle = random(10);
        if (twinkle === 1 && this.alpha > 0) {
          this.alpha -= 0.05;
        } else if (twinkle === 2 && this.alpha < 1) {
          this.alpha += 0.05;
        }
        ctx.globalAlpha = this.alpha;
        ctx.drawImage(
          canvas2,
          x - this.radius / 2,
          y - this.radius / 2,
          this.radius,
          this.radius
        );
        this.timePassed += this.speed;
      };
      for (var i = 0; i < maxStars; i++) {
        new Star();
      }
      function animation() {
        ctx.globalCompositeOperation = "source-over";
        ctx.globalAlpha = 0.5; //尾巴
        ctx.fillStyle = "hsla(" + hue + ", 64%, 6%, 2)";
        ctx.fillRect(0, 0, w, h);
        ctx.globalCompositeOperation = "lighter";
        for (var i = 1, l = stars.length; i < l; i++) {
          stars[i].draw();
          canvas2.style.cssText = "display:none";
        }
        window.requestAnimationFrame(animation);
      }
      animation();
    </script>
  </body>
</html>
